The use of dual fluid fuel injection systems in internal combustion engines is known in the art. A dual fluid injection system facilitates the delivery of both fuel and air into an engine cylinder during an engine operating cycle or “engine cycle”. During each engine cycle, there are thus generally two events which occur in a dual fluid fuel injection system: a fuel metering event, where a requisite quantity of fuel is typically provided by a fuel metering injector to a delivery or air injector; and an air injection or fuel delivery event, where compressed air is supplied through the delivery or air injector into the engine cylinder, with the metered quantity of fuel being delivered by and entrained in the supplied air.
Dual fluid fuel injection systems have proved efficient at providing good control over engine operation. The metering of the fuel, and the timing of the air injection or delivery event, can both be finely adjusted in response to engine load and other prevailing engine operating conditions. As a result, the engine can be operated efficiently, with engine emissions (particularly NOx and hydrocarbons) being kept low. Examples of dual fluid injection systems, as developed by the Applicant, are described in U.S. Pat. Nos. 4,800,862 and 6,564,770, the contents of which are hereby incorporated by reference. Such dual fluid injection systems implement one fuel metering event per engine cycle.
The fuel metering component, typically a pressure-time injector, within a dual fluid injection system is designed to operate most efficiently within a range of fuelling rates (generally measured in grams per second). This is known as the ‘dynamic range’ of the fuel metering injector. In practice, the design of the injector requires a degree of compromise to achieve an acceptable dynamic range. For example, in order for a particular engine to operate effectively under high load and/or speed conditions, the dynamic range of the fuel metering injector may need to be set such that engine operation at idle is less than ideal. Similarly, in certain circumstances the maximum power available from an engine may need to be limited to provide acceptable engine performance at low load and/or speed conditions.
The present invention seeks to provide a method of operation of a dual fluid fuel injection system which may be used in some circumstances to increase the dynamic range of the fuel metering injector.